Allegory, symbolism, critical analysis... sometimes we just overdo it. Books have been used to propel ideas through subtle references and metaphor, and often the reader easily gets the point: Narnia is about Christianity for instance, but no one would really describe this as subtle.
Sometimes a classic book becomes divorced from its historical context over the years. Or the intent of the author is ignored in favour of some other unintentional message, or sometimes the public as a whole just doesn't really 'do' nuance. And of course occasionally the movie is just a bad adaptation.
It raises an interesting question of who decides what the meaning of a work truly is: is it the person who planned and executed the storytelling or is it the person who in the end will pay for and consume the work? It can be argued that as the consumer and members of the court of public opinion that we decide what the true meaning is. But when a person takes the time to write the damn thing we should at least assume they get a pretty big say in the matter.
But sometimes a work is created to carry a specific meaning and instead of attributing some outside truth, we instead suggest it's about ghosts or some other nonsense.
10. The Prince Is Not Actually Machiavellian
Because of this book any act of social of political subterfuge in the name of personal advancement is characterised as 'Machiavellian'. The Prince acts as a guide on removing empathy and kindness from political practices, and discusses the best means of consolidating power and fooling the population. Even in the modern era it is relevant in its instruction, except it's all a pisstake.
Similar to Johnathan Swift's 'A Modest Proposal' (which isn't actually about eating children) the book is a satire, pointing out the flaws with the contemporary regime of 1500s Italy by posing as an educational text. Unlike Swift's guide on the preparation and eating of the poor, however, many people have tended to take the book at face value and see it as an actual guide on gaining power.
At this point in European history the upper class were given education on some themes that Machiavelli discusses, just without the sarcasm.
Rarely for its time and genre, it was written in vernacular Italian rather than Latin, leading some theorists to suggest Machiavelli may also have aimed to show the proletarian class the injustices present in the regime. It doesn't help when everyone misses the joke and just takes you seriously though.